A generator motors when the mechanical input power to its shaft is lost while connected to the power system. Assuming that the generator field is unaffected, the generator becomes a synchronous motor driving the prime mover at synchronous speed. If field excitation is also lost, the generator may lose synchronism and become an induction motor. This chapter will consider only motoring conditions with the field intact and the turbine generator at synchronous speed. Loss-of-field is the subject of a subsequent chapter.